The present invention relates generally to glass-ceramics containing forsterite (Mg2SiO4) as the major crystalline phase. More particularly, the glass-ceramics have a small crystal size to make the glass-ceramic material optically transparent and are doped with chrome (Cr) at relatively high levels, which is useful as gain media, in optical amplifiers and/or laser pumps. The term xe2x80x9cgain mediaxe2x80x9d refers to an optical component that produces optical fluorescence and is capable of amplifying an optical signal in the same wavelength range as the optical fluorescence. The invention also relates to a more formable glass-ceramic composition that is useful for drawing optical fibers.
Recently, researchers have concentrated much effort to develop transparent glass-ceramics as hosts for transition metal ions. Transition metals have been used as optically active dopants in crystalline hosts because they fluoresce in the near infrared (xcx9c1000 nm to xcx9c1500 nm) region. Given the useful wavelength range and relatively wide bandwidth of many transition-metal dopants, much interest has arisen for their use in optical telecommunication applications. The current optical telecommunication medium is glass-based optical fiber. Inclusion of transition metal dopants into glasses, however, has unfortunately not produced fluorescence performances as good as in crystalline materials. The performance of transition metal ions tends to degrade in amorphous hosts, where the crystal field strength is much smaller than in even single-crystal hosts.
Suitable glass-ceramic hosts, therefore, must be tailored such that transition elements will preferentially partition into the crystal phase. Some of these glass-ceramics have come from compositions such as those discussed in co-pending U.S. patent application Ser. No. 09/686,418, entitled TRANSPARENT FORSTERITE GLASS-CERAMICS, by George H. Beall, which relates to a family of glass compositions based in the K2Oxe2x80x94MgOxe2x80x94Al2O3xe2x80x94SiO2 system, or in co-pending U.S. patent application Ser. No. 09/686,564, entitled TRANSITION-METAL GLASS-CERAMIC GAIN MEDIA, by George H. Beall et al., which relates to transition-metal-doped glass-ceramic materials used as gain media or pump laser fiber in optical amplifiers and lasing mechanisms. The entire contents of both of these applications are incorporated herein by reference.
Glass-ceramics are polycrystalline materials formed by a controlled crystallization of a precursor glass. In general, the method for producing such glass-ceramics customarily involves three fundamental steps: first, melting a glass-forming batch containing the selected metallic oxides; second, cooling the melt to a temperature at least below its transformation range, while simultaneoulsy forming a glass body of a desired geometry; and third, heating the glass body to a temperature above the transformation range of the glass in a controlled manner to generate crystals in situ. To develop nuclei in the glass, the glass will be heated initially to a temperature within or somewhat above the transformation range for a period of time. Thereafter, the temperature will be raised to levels approaching, or even exceeding, the softening point of the glass to grow crystals from the nuclei. The resulting crystals are typically uniformly distributed and fine-grained. Internal nucleation permits glass-ceramics to have favorable qualities such as a very narrow distribution of particle size and a highly uniform dispersion of crystals throughout the glass host.
Transparent glass-ceramics are known in the art, with the classic study relating to transparency being authored by G. H. Beall and D. A. Duke in xe2x80x9cTransparent Glass Ceramics,xe2x80x9d Journal of Material Science, 4, pp. 340-352 (1969). Glass-ceramic bodies will display transparency to the human eye when the crystals present therein are considerably smaller than the wavelength of visible light. In other words, transparency typically results from crystals less than 50 nmxe2x80x94preferably as low as 10 nmxe2x80x94in size. Transparency in glass-ceramics, alternatively, can also be produced with crystals larger than 50 nm if the crystal birefringence and the index of refraction mismatch between the crystal phase and the glassy phase are both low. Transparent glass-ceramics, doped with transition elements can combine the optical efficiency of crystals with the flexibility of the forming of glass. For example, both bulk (planar substrates) and fiber forms can be fabricated from these glass-ceramics.
Forsterite is an orthosilicate with two distinct octahedral sites, both occupied by Mg2+, and one tetrahedral site occupied by Si4+. All three of these cation sites are highly distorted. The octahedral sites have mirror and inversion symmetries and the tetrahedral site is pyramidally distorted. It has been shown that chromium ions can enter the forsterite structure as Cr3+ in the octahedral sites, and as Cr4+ in the tetrahedral sites. The Cr4+ ion has further been identified as the key lasing ion in single crystals responsible for the major portion of luminescence over the wide band extending from about 900 nm to about 1400 nm and centered at about 1175 nm. (A shoulder on the band near 1000 nm is attributed to Cr3+ ions).
Chromium-doped forsterite, a transition-metal-silicate crystal species, has demonstrated the ability to produce optical gain over a broad portion of the near infrared spectrum and has been fabricated as single-crystal tunable or femtosecond lasers. In the late 1980s, it was discovered that single crystals of chromium-doped forsterite could be used as a laser material in the 1210 nm to 1260 nm region. Further work, determined that the active ion was Cr4+, a rare valence state of chromium, and that strong luminescence and tunable laser action could be produced in the broad spectral region from about 1100 nm to about 1400 nm, and perhaps even deeper into the infrared.
In the past, however, the maximum gain (complete population inversion) for a material with about 25% crystalline forsterite particles was calculated, using published optical constants for forsterite, to be about only 240 dB/m. To increase the overall fluorescence within forsterite-containing glass-ceramic materials, a greater crystalline yield of forsterite needs to be achieved. The present invention provides a method and glass composition that satisfies this need.
The present invention resides in part in transparent glass-ceramics that have a level of nanocrystallinity of at least about 30% forsterite components by weight when at a relatively low liquidus temperature of about 1525-1500xc2x0 C. or less. This is a higher yield of forsterite component and crystals than was previously achievable, on a sustainable basis, at such relatively low temperatures. The predominant forsterite crystal phase in the glass-ceramic is doped with chromium at levels higher than that which was previously practical to perform forming operations such as drawing optical fibers. In part, the key to improvements in formability with greater crystallinity of forsterite involves the addition of Na2O as a major ingredient (over about 3% by weight), coupled with increased levels of titania (xe2x89xa75% by weight) in an original glass composition. The effect of increased amounts of Na2O, in replacing K2O amounts, is to lower the liquidus temperature of forsterite at a given level of theoretically attainable forsterite in the original glass composition. Moreover, increased sodium levels lead to higher allowable MgO levels, which in turn, increase the solubility of Cr4+ ions in the glass. As a result, more Cr4+ ions are then available to be incorporated into forsterite nanocrystals for greater luminescence. In short, the invention provides better physical flexibility in forming glass-ceramic objects, and better overall fluorescence and performance in optical gain, due to greater crystallinity.
The glass-ceramics of the present invention have a glass composition, in weight percent on an oxide basis of about 40% to 60% SiO2; 10% to 25% Al2O3; 18% to 30% MgO; 3% to 10% Na2O; 0% to 10% K2O;  greater than 5% to 15% TiO2. Preferably, the composition consists essentially of about 43% to 55% SiO2; 11% to 16% Al2O3; 20% to 26% MgO; 3.5% to 6.5% Na2O; 3.0% to  less than 8.0% K2O; 5.5% to 9.0% TiO2. To obtain optical activity, i.e., fluorescence, over the infrared telecommunications transmission wavelength range of about 900 nm to about 1400 nm, the present forsterite glass-ceramics are doped with up to about 1.3% chromium oxide by weight, and preferably with about 0.05% to about 0.75% chromium oxide.
The present invention also encompasses a method of dissolving at least 30% by weight of forsterite components in a glass-ceramic. The method comprises providing a R2Oxe2x80x94MgOxe2x80x94Al2O3xe2x80x94SiO2 glass composition, wherein R is an alkali ion, containing, in weight percent, at least about 3% of Na2O coupled with greater than 5% of TiO2, and melting the glass at a temperature between about 1575xc2x0 C. to about 1650xc2x0 C. Preferably the temperature ranges from about 1590xc2x0 C. to about 1630xc2x0 C. Then, heat treating the glass according to a ceramming schedule to precipiate crystals, and achieve at least 30% by weight of forsterite component in the glass-ceramic at a liquidus temperature of about 1525xc2x0 C. or below.
The present invention further includes an optical fiber and/or a gain medium comprising a transparent glass-ceramic containing a crystallinity of at least about 30% by weight of forsterite components at a liquidus temperature of about 1525xc2x0 C.xc2x15xc2x0 C. or less.
Additional features and advantages of the invention will be described in the detailed description that follows.